Scanned color separations are used as artwork in the reproduction technique to produce offset- or relief printing plates. The color separations are proofed, before the exposure of the printing plates, using a color proofing process to determine if the later printing result represents an accurate tonal reproduction of the original.
Radiation sensitive recording materials which utilize differences in the tackiness of exposed and unexposed areas are used to form images in color proofing processes. A reproduction process is described for example, in Berg, German Patent 1210321 (U.S. Pat. No. 3,060,024); Chu, German Patent 1904058 (U.S. Pat. No. 3,620,726); Boyd, German Patent 1904059 (U.S. Pat. No. 3,582,327); Chu, German Patent 2004214 (U.S. Pat. No. 3,649,268); Buzzell, U.S. Pat. No. 4,356,253; and Bauer, European Patent 0243933 (U.S. Pat. No. 4,892,802), in which a tacky, photopolymerizable recording material, consisting of a support and a polymerizable layer that contains at least one addition- polymerizable monomer and a photopolymerization initiator, is hardened upon imagewise exposure. The exposed areas loose their tackiness in this case. The latent image is made visible by applying suitable toning materials, which adhere only to the unexposed, tacky areas. Excess toner can be removed from the exposed, non-tacky areas after the application. With this process, one obtains positive toned images of the artwork that look like pictures that were produced using printing inks.
Negative images of the artwork are obtained using light sensitive materials such as those described in Abele, U.S. Pat. No. 4,243,741; Abele, U.S. Pat. No. 4,346,162; and Grossa, U.S. Pat. No. 4,604,340. These recording materials contain as light sensitive components either a dihydropyridine compound or a system from a dihydropyridine compound and a hexaarylbisimidazoyl compound.
Application of toner materials that consist mainly of fine particle powders can be carried out by dusting the imagewise exposed surface as described, for example, in Manger, U.S. Pat. No. 4,215,193, and Gray, U.S. Pat. No. 3,909,282. In another embodiment, the toner can be loosely attached to a special carrier and then transferred to the imagewise exposed surface by contacting it with this carrier. Such pigmented carrier layers are described in Frohlich, U.S. Pat. No. 4,806,451, and Metzger, Canadian Patent 2,032,051.
The radiation sensitive recording materials consist usually of a support, a radiation sensitive layer and a coversheet. This coversheet is removed and the material is laminated to a receptor before the imagewise exposure takes place. The support can be removed before or after the exposure, depending on the material.
A recurring problem with this process is the inability to separate the coversheet cleanly from the radiation sensitive layer before lamination to the receptor, because the adhesion of the radiation sensitive layer to the coversheet is too high. Either the radiation sensitive layer separates partially from the support or some areas of the layer are pulled off by the coversheet. Naturally, such damaged layers can not be used for the color proofing process.
The adhesion balance between the radiation sensitive layer and the support can be improved somewhat by the addition of aliphatic polyesters with molecular weights between 1500 and 40000, such as described in Bauer, U.S. Pat. No. 4,326,010. However the difficulties described above can still occur.
Special polyorganosiloxane compounds are used because of their adhesion properties in release layers such as those for the supports of tacky labels. These are described in Weitemeyer, U.S. Pat. No. 4,678,846; Weitemeyer, European Patent 284,863; and Weitemeyer, U.S. Pat. No. 4,963,438. Such release layers, however, can not be compared with the tonable, radiation sensitive layers used in the described color proofing process, because they have to be removable from both the coversheet and the support, while still demonstrating good adhesion to the receptor as well as to the other recording layers.
Polydimethylsiloxane is known from Nakamura, German Patent 3,337,303, as a material used to prevent the sticking of a photomask to photoresist materials. It is however unsuitable for use in the named tonable recording materials, since layers that contain polydimethylsiloxane, do not adhere sufficiently to the other recording layers.
Special silanes are suitable as additives for photoresist films, having a reduced residual layer after development as described in Lauke, European Patent 329,009. This does not suggest, however, that silanes can be used for recording materials for color proofing processes, which have to have very specific adhesion balances.
Accordingly, one purpose of this invention is to eliminate the above-described problems in the color proofing process by optimizing the adhesion balance of a tonable, radiation sensitive recording layer to a coversheet and a support, in a way that allows selective removal of the coversheet first but the support is also removable. This must be achieved without damaging the recording layer during either removal process and without influencing the adhesion of the recording layer to the receptor or to other recording layers. In addition, a defect free toner application to the imagewise exposed layer must be assured.